Field of the Invention
The present invention relates to a polymer compound containing a repeating unit sensitive to a high energy beam, a positive resist composition containing the polymer compound, a laminate containing a resist film formed from the positive resist composition, and a resist patterning process using the positive resist composition.
Description of the Related Art
A finer pattern rule has been recently required for integrated circuits with higher integration. To process a pattern with a size of 0.2 μm or less, a chemically amplified resist composition, which uses an acid as a catalyst, has been mainly used. In this process, high energy beams such as ultraviolet ray, far ultraviolet ray, extreme ultraviolet ray (EUV), and electron beam (ER) are used as an exposure light source. Especially, the electron beam lithography, which is employed as ultrafine processing technique, is also indispensable for processing a photomask blank in producing a photomask for semiconductor manufacture.
A resist composition used in such a photolithography can be classified into a positive type, in which an exposed part dissolves to form a pattern, and a negative type, in which an exposed part remains to form a pattern. These compositions are appropriately selected according to the form of the resist pattern to be required.
In general, the electron beam lithography uses an electron beam. In the case of positive resist, the resist film is sequentially irradiated with an electron beam having a fine surface area except for a region to be left, without using a mask. In the case of negative resist, the region to be left of the resist film is sequentially irradiated. Consequently, this method needs to sweep across the finely divided region on the surface to be processed and thus takes more time than one-time exposure using a photomask. To keep the throughput high, this method requires a sensitive resist film. In the photomask blank processing, which is particularly important use, some photomask substrates are coated with a surface material that easily affects pattern profile of the chemically amplified resist film, like a chromium compound film such as chromium oxide film. To keep high resolution and profile after etching, it is important to keep the pattern profile of the resist film rectangular, regardless of the type of the substrate. Moreover, it is also important that line edge roughness (LER) is small.
Incidentally, control of the resist sensitivity and the pattern profile has been improved by various methods, for example, by selecting or combining materials used for the resist composition and process conditions. One of the improvement methods is to reduce acid diffusion, which significantly affects resolution of the chemically amplified resist film. The photomask processing requires that the profile of the obtained resist pattern is not changed depending on the time between exposure and baking. This time-dependent change is mostly caused by diffusion of an acid generated by exposure. The acid diffusion significantly affects, not only the photomask processing, but also sensitivity and resolution of a usual resist composition. Thus, many studies have been made on this problem.
Patent Document 1 and Patent Document 2 disclose examples of controlling the acid diffusion by using a bulky acid generated from an acid generator to reduce LER. However, this acid generator is still insufficient for controlling the acid diffusion. Thus, it is desired to develop an acid generator that can more greatly reduce the diffusion.
Patent Document 3 discloses an example of controlling the acid diffusion by incorporating a repeating unit having a sulfonium structure that generates a sulfonic acid by exposure into a resin used in the resist composition. Such a method of incorporating a repeating unit that generates an acid by exposure into the base resin is effective in obtaining a pattern with small LER. However, the base resin incorporated with the repeating unit that generates an acid by exposure can have a problem of solubility in an organic solvent, depending on the structure and the introducing ratio.
Besides, polymers having many aromatic skeletons with acidic side chains, such as polyhydroxystyrene, have been favorably used for a resist composition for KrF excimer laser, but not for ArF excimer laser because they have large absorption of light about 200 nm wavelength. However, these polymers are important to the resist compositions for electron beam and for EUV, which are effective in forming a finer pattern than a processing limit of the ArF excimer laser, in that the polymers have high etching resistance.
A base resin mainly used in the positive resist composition for electron beam or EUV contains an acid-labile protective group that masks an acidic functional group of a phenolic side chain. This protective group deprotects by an acid catalyst generated from a photo acid generator by irradiation with a high energy beam, thereby causing the base resin to be solubilized in an alkaline developer. As the acid-labile protective group, a tertiary alkyl group, a tert-butoxycarbonyl group, an acetal group, and the like have been mainly used. The use of a protective group that requires a relatively low activation energy for deprotection, such as the acetal group, can advantageously provide a sensitive resist film. However, if the diffusion of generated acids is insufficiently controlled, deprotection reaction takes place even in an unexposed part of the resist film, thereby causing problems of lowering LER and decreasing in-plane uniformity of the pattern line width (CDU, Critical Dimension Uniformity).
In addition, Patent Document 4, which discloses a resist composition containing a sulfonium salt capable of generating a strong acid such as a fluorinated alkanesulfonic acid and a resin having a repeating unit containing an acetal group, has a problem of forming a pattern with large LER. Patent Document 5, which discloses incorporating a fluorinated alkanesulfonic acid into the resin to reduce the acid diffusion, also fails to solve the same problem.
This pattern with large LER is supposed to be caused by too strong acidity of the fluorinated alkanesulfonic acid relative to deprotection of the acetal group, which requires a relatively low activation energy for deprotection. In other words, a trace of acids diffused into the unexposed part advances the deprotection reaction even if the acid diffusion is controlled. The same problem arises in the case of using a protective group that requires higher activation energy than the acetal group, such as a tertiary alkyl group and a tert-butoxycarbonyl group. Thus it is desired to solve this problem.